Separation of particulate solids of varying densities in a fluidized bed

ABSTRACT

A particulate feedstock is separated into a light fraction and a heavy fraction by feeding it into a fluidized bed of particulate material whose particle size and specific gravity, relative to the particle sizes and specific gravities of said light and heavy fractions, are such that one of these fractions readily stratifies into an easily removable layer in the bed while the other fraction tends to remain intermixed with the bed material rather than stratifying. Said other fraction is removed as product from the bed while it is in said intermixed condition.

United States Patent 1 Weintraub et al.

[ 51 Nov. 27, 1973 SEPARATION OF PARTICULATE SOLIDS [54] OTHERPUBLICATIONS 0F VARYING DENSITIES IN A FLUIDIZED BED FourthInternational Coal Preparation Congress,

1962, pp. 405407. [75] Inventors: Murray Weintraub; Albert W. 3.-- .WDeurbmuck, both of Pittsburgh, Primary Examiner-Frank W. Lutter [73]Assignee: The United States of America, as Assistant ExaminerwilliamCllchlinski,

represented by the Secretary of the AttorneyEmest S. Cohen and M. HowardSilverstein eri W. l. .2

22 F1 d Dec 16 1970 [57] ABSTRACT 1 l e A particulatefeedstock isseparated into a light frac- [21] Appl. No.: 98,509 tion and a heavyfraction by feeding it into a fluidized bed of particulate materialwhose particle size and [52] us. CL 209/474 specific gravity, relativeto the particle sizes and spe- [51] Int Cl 33.03) 3/04 cific gravitiesof said light and heavy fractions, are [58] i 4 11 502 such that one ofthese fractions readily stratifies into 4 172 5 an easily removablelayer in the bed while the other fraction tends to remain intermixedwith the bed ma- [56] References Cited terial rather than stratifying.Said other fraction is re- UNITED STATES PATENTS moved as product fromthe bed while it is in said intermixed condition.

l,291,137 1/1919 Reed 209/474 3,349,912 10/1967 Eveson et a1. 209/4742,303,367 12 1942 Kendall et a1 209/466 x 2 Claims, 2 Drawing g sFOREIGN PATENTS OR APPLICATIONS 1,058,078 3/ 1954 France 209/474 F E E DAIR 4\ i L 3- FLUIDIZING AIR i P RODUCT PRODUCT PAIENIEDum/27 ms3.774.759

FLUIDIZING AIR PRODUCT PRODUCT FEED AIR 4a RECYCLE 1 I fl zrglmzme mi 05 PRODUCT PRODUCT Fla 2 INVENTO/PS ATTORNEYS SEPATKUN OF PARTHCULATESOLIDS F VARYHNG DENSITES IN A FLUIDIZED BED This invention relates tothe separation of particulate solids of varying densities from oneanother.

The separation of particulate solids from one another by stratificationof the heavy and light fractions in a fluidized bed is a well knowntechnique. To enhance control of the separation, it is also well knownto employ certain additional particulate materials such as fine sand ormagnetite as the fluidized bed material. In such an operation, the sandor magnetite is maintained in a fluidized state after which aparticulate feedstock such as as-mined crushed coal is injected.impurities in the coal then sink to the bottom while the lighterpurified coal particles float to the top. This latter technique is aparticulately effective separation technique, except that as thefeedstock decreases in size, it has heretofore been observed that ittends to mix with the bed material rather than readily stratifying intoa plurality of layers. Thus, with feedstocks composed of relativelysmall particles, separation by stratification is impractical, time-wise,and is sometimes apparently impossible to achieve. Further, the lowerair velocities required for stratification separation with such smallerparticles often times cannot be successfully maintained because of thepresence of larger particles in the feedstock which tend to inhibitsmooth fluidizationat low gas velocities. As a result, heretoforefluidized bed separations have not been employed to separate particulatesolids into two fractions wherein'the feedstock particles are so smallthat the feedstock tends to intermix with the bed material rather thanreadily separating into a plurality of layers. This is particularlymanifested in the stratification separation of impurities from coalparticles having a diameter of less than one-fourth inch.

We have now developed a fluidized bed separation system for a feedstockhaving-relatively small particles which do not readily stratify into aplurality of layers within the bed material. in arriving at ourdiscovery, it was first observed that with a relatively fine feedstockof, for example, impure coal, although the lighter (purified coal)particles tended to remain intermixed with the magnetite bed material,the heavier (impurity) fraction would readily stratify to the bottom ofthe bed. This observation is congruous with the discussion on theintermixing phenomena presented in the Fourth International CoalPreparation Congress, 1962, pages 405-407. Therein the researchersreported that complete intermixing of a feedstock fraction with thefluidized bed material would occur when the ratio of the feedstockparticle diameter (11,) to the fluidized bed material particle diameter(d equaled the inverse proportion to power 3/2 of the ratio between thespecific gravity (6,) of the feedstock fraction and the specific gravity(8 of the bed material particles, i.e., al /d (8J8 These sameresearchers also observed (page 406) that even when the feedstockparticles are somewhat larger in size than the size necessary forcomplete mixing in accordance with this formula, there is stillsufficient mixing to prevent a practical and economical stratificationseparation.

Accordingly, in the practice of the present invention the relativeparticle sizes and specific gravities of (a) the bed material and'(b)the light and heavy fractions of the feedstock are such that one of thefractions readily stratifies into an easily-removable layer while theother fraction tends to remain intermixed with the bed material. Saidother fraction is then removed, as product, from generally the middle ofthe bed while it is still intermixed with the bed material. To carry outthe operation, both the feed conduit and product withdrawal conduits areadjustable with respect to different sections or portions of thefluidized bed. In particular, coal of less than one-fourth inch indiameter is readily separated from heavy impurities by the system of thepresent invention.

As used throughout the specification and claims, the terms mixed andintermixed mean that the feedstock fraction is essentially in anon-stratified state within the fluidized bed material.

It is therefore an object of the present invention to separate aparticulate solid material into a light and heavy fraction in afluidized bed wherein one of these fractions readily intermixes with thebed material rather than stratifying.

A further object is to separate coal from impurities in such a system.

A still further object is to provide apparatus for such a separation.

Other objects and advantages will be obvious from the following moredetailed description of the invention in conjunction with drawings inwhich:

FIG. l is a schematic'side view of an apparatus suitable for carryingout the present invention; and

FIG. 2 is an alternative embodiment.

In the practice of the present invention the equipment employed to bringabout fluidized bed separation of particulate solids is basically thesame as that heretofore employed in this art. The essential differenceis that only one means is provided for removing stratified product fromthe bed. The other product withdrawal means is located generally towardthe middle section or portion of the bed. 7

Referring now to FIG. ll, reference numeral 1 designates a cylindricalseparation chamber having an air distributor plate 2 which supports thefluidized bed. Conduit 3'introduces fluidizing air below plate 2.Fluidizing gas velocities are generally the same as those heretoforeemployed in the separation-by-stratification art. in such prior artoperations, the bed material is desirably maintained in a fluidized butquiet condition with a minimum amount of ebullition. Such smoothfluidization prevents the feedstock particles from being agitated withinthe bed material, and thus prevents intermixing and enhancesstratification thereof. In the present invention, these same fluidizingvelocities are employed except that the need for substantiallynonbubbling conditions is not as critical since there is no concern ifone of the feedstock fractions intermixes with the bed material.

A conduit 4 conveys bed material to the chamber 1 so that a fluidizedbed 5 can be established therein. Thereafter, the particulate feedstockto be separated intoa light and heavy fraction is introduced into thechamber through conduit 4 together with make-up bed material, ifdesired, to replace bed material removed during product withdrawal. Byemploying a vertically and radially adjustable feed conduit,the'feedstock can be admitted into the bed, at equilibrium, at a pointmost closely approximating the composition of the incoming feedstock,which results in a more stable and efficient operation. This isanalogous to the variable feed plate selection in the operation of adistillation column.

For any particular feedstock, the respective particle sizes and specificgravities of the light and heavy fractions thereof relative to theparticle size (d) and specific gravity (8) of the bed material are suchthat one of these feedstock fractions will readily mix with the bedmaterial rather than stratifying, while the other fraction will readilystratify. If the size (d of one of the feedstock fractions is such thatit is less than times the size necessary for complete intermixing withthe bed material in accordance with the formula 1 bed material (51m:material r) then said one fraction will tend to remain substantiallyintermixed with the bed material. The intermixing becomes greater andthe advantages of this invention more pronounced when the particle sizeof the to-beintermixed fraction is less than 4 times the size necessaryfor complete intermixing. Stratification of the tobe-intermixed fractionwill, of course, occur among some of the particles larger than the sizenecessary for complete interrnixing, but this does not detract from theoperability of the present invention.

With regard to the to-be-stratified fraction of the feedstock, in orderto maximize stratification, its particle size is preferably at least 10times its theoretical size necessary for complete intermixing inaccordance with the above formula, although adequate stratification andsignificant separations can be achieved when it is at least 4 times saidtheoretical size.

As can be seen from the above formula, when both the light and heavyfractions in the feedstock are ap-" proximately the same size, theoperation can be carried out so that only the light fraction intermixeswith the bed material; but intermixing of only the heavy fraction cannotbe attained under these conditions. However, in a feedstock wherein theheavy fraction is much finer than the light fraction, then there can bean operation wherein the light fraction stratifies while the heavyfraction intermixes. With regard to the drawings, for the sake ofconvenience, the intermixed fractionwill be referred to as the lightfraction, and the heavy fraction as the stratified layer.

Referring again to FIG. 1, a vertically adjustable conduit 6 projectsinto the bed, and has an' opening at the bottom of the bed to remove thestratified heavy fraction of the feedstock together with some bedmaterial as the bottom layer 5a thereof. Vertically adjustable conduit 7projects into the central portion of the bed intermediate the top andbottom sections, and removes the intermixed light fraction.

Both conduits 6 and 7 extend through seals 8 in plate 2. Other seals(not shown) similar to seals 8 are radially disposed and plugged inplate 2 so that withdrawal conduits 6 and 7 may be adjusted radially aswell as vertically, if desired. Conduits 6 and 7 convey product toseparation chambers (not shown) of a design heretofore employed in theart so that bed material can be separated therefrom and recycled tochamber 1 through conduit 4. Such separation chambers may simply bescreening devices or magnetic separators in situations where the bedmaterial is, for example, magnetite while the feedstock is nonmagnetic.

Product quality is determined by residence time within vessel 1, andthis is controlled by the means heretofore employed to adjust rates offlow from fluidized beds.

Referring now to the alternative embodiment shown in FIG. 2, referencenumeral 11 designates a rectangular trough having an air distributorplate 12 supplied with fluidizing air through conduit 13. Bed materialand, subsequently, the feedstock are admitted through laterally andvertically adjustable conduit 14 to establish fluidized bed 15.

An adjustable orifice 16 is located at the bottom of an end wall 11a oftrough 11 to allow the stratified heavy fraction to be removed from thebed as the bottom layer 15a thereof. In the middle of the same end wall11a is located another adjustable orifice 17 to allow the intermixedlighter fraction of the feed to be removed from generally the middle ofthe bed 15 intermediate the top and bottom sections or portions thereof.Recycled bed material can be admitted with the feedstock through conduit14 or through a vertically adjustable recycle conduit Residence time ofthe feedstock can be adjusted by the positions and areas of orifices i6and 17 in end wall 11a, by the lateral position of inlet conduit 14; orby inclination of the vessel by optional pivot means 18.

Other fluidized bed mechanisms such as splitter plates and baffles,mechanical rakes and vibrators for facilitating moving can obviously beemployed in the system.

The following example illustrates the effectiveness of the process ofthe present invention:

EXAMPLE An impure ground coal (30 percent impurities; coal, 1.35 sp.g.;impurities 2.6 sp.g.) having a particle size of 50 percent in the rangethree-eighths to one-fourth inch, 25 percent in the range 6 to 8 U.S.S.mesh and 25 percent in the range 14 to 30 U.S.S. mesh was fed into afluidized bed of magnetite (200-300 U.S.S. mesh) being fluidized withair at 2.1 feet per minute. The coal was maintained in the bed for 5minutes. Prior to product withdrawal, the bed consisted of 1.8 percentby weight of coal feedstock and 98.2 percent magnetite.

Purified coal then drawn off at a bed height of 60 percent contained 2.9percent impurity, and was equal to 83.7 percent of the potentially cleancoal in the feedstock. When the experiment was repeated and the coalproduct was drawn off at a bed height of 66 percent, 90.7 percent of thepotentially clean coal in the feedstock was recovered containing 3.8percent impurity.

Although the present invention has been described with regard to aparticular fraction such as the light fraction which tends to intermixwith the bed material rather than stratifying, it should be understoodthat there may be present in the feedstock substantial amounts of largerparticles in the light fraction which readily stratify to the top layerin the fluidized bed. Under these circumstances, it may be advantageousto provide top layer removal means in conjunction with the means toremove particulate matter from the bottom and middle of the bed.

What is claimed is:

l. A process for the separation of a particulate feedstock into a lightfraction and a heavy fraction consisting essentially of a. feeding saidfeed stock into a fluidized bed of particulate material whose particlesize and specific gravity, relative to the particle sizes and specificgravities of said light and heavy fractions, are such that one of saidfractions readily stratities as a top or bottom layer adjacent said bed,and the other of than four times the theoretical size of said otherfraction as calculated from the formula d d (8 /8,) and wherein therelationship of the particle size (d and specific gravity (8 of said onefraction of the particle size (d and specific gravity (8 of said bedmaterial is such that the particle size of said one fraction is at least10 times the theoretical size of said one fraction as calculated fromthe formula 11;, d (8 /8 2. The process of claim 1 wherein saidfeedstock is impure coal and said light fraction is purified coal.

1. A process for the separation of a particulate feedstock into a lightfraction and a heavy fraction consisting essentially of a. feeding saidfeed stock into a fluidized bed of particulate material whose particlesize and specific gravity, relative to the particle sizes and specificgravities of said light and heavy fractions, are such that one of saidfractions readily stratifies as a top or bottom layer adjacent said bed,and the other of said fractions readily intermixes with said bedmaterial as a non-stratified fraction throughout said bed; b. removingsaid stratified layer away from said bed; c. removing said otherfraction from within said bed while said other fraction isnon-stratified and intermixed with said bed material; and wherein therelationship of the particle size (d1) and specific gravity ( delta 1)of said other fraction to the particle size (d2) and specific gravity (delta 2) of said bed material is such that the particle size of saidother fraction is less than fouR times the theoretical size of saidother fraction as calculated from the formula d1 d2( delta 2/ delta1)3/2, and wherein the relationship of the particle size (d3) andspecific gravity ( delta 3) of said one fraction of the particle size(d2) and specific gravity ( delta 2) of said bed material is such thatthe particle size of said one fraction is at least 10 times thetheoretical size of said one fraction as calculated from the formula d3d2( delta 2/ delta 3)3/2.
 2. The process of claim 1 wherein saidfeedstock is impure coal and said light fraction is purified coal.